1. Field of the Invention
The present invention relates to single-lens reflex cameras, and more particularly to a reflex apparatus disposed in such cameras for reflecting a scene image to an optical viewfinder to enable the operator to compose the picture and focus the lens.
2. Description of the Prior Art
The single-lens reflex camera has long been popular among photographers on account of, among other features, the capability for the operator to observe an exact representation of the image prior to an exposure by means of an optical viewfinder. The elimination of parallax error ensures accurate scene composition and permits the operator to corroborate the lens focus in real-time.
The prior art single-lens reflex camera comprises several components in a well-known design. A reflex mirror is mounted on a hinge, which allows for rotational movement between two positions. The first mirror position is the observation position, wherein the reflex mirror intersects the principal optical light path between the objective lens and the image-sensing device at an angle that is oblique to said light path. In the observation position, the reflex mirror reflects the principal optical light path upward into the optical viewfinder, enabling the operator to observe the scene image through the same perspective and focus as the image-sensing device. To affect an exposure, the operator actuates the shutter-release button. Immediately prior to the opening of the shutter, the reflex mirror is retracted out of the principal optical light path, pivoting about the hinge until it reaches the second mirror position, henceforth referred to as the photographic position. In the photographic position, the reflex mirror is situated parallel to the focusing screen to allow light from the objective lens to fall incident upon the image-sensing device, which may be of the photographic film or electronic (digital) sensor type. While in the photographic position, the reflex mirror also serves to block stray light from entering the chamber through the optical viewfinder. Once the predetermined exposure time value has lapsed, the shutter closes and the reflex mirror is returned to the observation position, wherein the operator is able to resume monitoring the scene image through the optical viewfinder.
The prior art single-lens reflex camera suffers from inherent design limitations. The movement of the reflex mirror between the observation and photographic positions creates vibrations in the camera body that can produce motion-blur artifacts in acquired images. The noises of the reflex mirror striking the terminus positions during operation prevent the practical use of single-lens reflex cameras in circumstances when silence is required. Additionally, substantial space is required forward of the reflex mirror to prevent a collision with the rearmost element of the photographic objective lens when the mirror is swung upward, which contributes to the size of the camera. Furthermore, the mechanism for inserting and retracting the reflex mirror from the principal optical light path requires precise manufacturing tolerances and a large number of moving parts, which makes it especially susceptible to damage from shocks and drops. Ever-increasing image acquisition rates introduce significant mechanical difficulties for the operation of the reflex mirror mechanism. Current single-lens reflex cameras are capable of acquiring up to 11 images per second; the mechanical stresses associated with swinging a reflex mirror from the observation position to the photographic position and back 11 times a second precipitate issues such as frictional wear and material fatigue.
One prior art attempt to address these limitations has been to replace the movable reflex mirror of a conventional single-lens reflex camera with a stationary, half-silvered pellicle mirror (Pellix, Canon Inc., 1965). The pellicle mirror is permanently affixed in the aforementioned observation position, likewise intersecting the principal optical light path at an oblique angle. Due to the semi-transparency of the pellicle mirror, a portion of the light passing through the lens system is reflected to the optical viewfinder while the remaining portion of light is transmitted to the image-sensing device. This design enables the operator to continuously monitor and compose the scene image even while affecting exposures, since the mirror does not swing up and induce a viewfinder blackout. However, since the pellicle mirror splits the available incoming light into two parts, it cannot provide the viewfinder with an optimal reflection of the scene image, nor can provide the image-sensing device with an optimal transmission of the light during an exposure. Nevertheless, the use of a pellicle mirror mitigates some of the limitations of the conventional single-lens reflex camera by eliminating the complex moving mirror mechanism, along with the noise and vibration consequent to its action.
It is now prudent to discuss the topic of “smart glass,” as it is an integral component of the present invention. Smart glass, also commonly known as “switchable glass,” “intelligent glass” or “privacy glass,” refers to a class of materials whose light transmission properties change reversibly in response to the application of electrical voltage. Electrochromics (ECs), suspended particle devices (SPDs), and polymer dispersed liquid crystals (PDLCs) are examples of technologies employed in smart glass. Some types of smart glass are characterized by their ability to switch reversibly between transparent and opaque states in response to an applied electrical voltage, and will henceforth be referred to as “T/O smart glass.” Advancements in material research pertaining to transition-metal hydrides have led to the development of reflective hydride electrochromics, which become specularly reflective, as opposed to opaque. This technology pertains to a subcategory of smart glass that has the ability to switch reversibly between transparent and reflective states in response to an applied electrical voltage, and will henceforth be referred to as “T/R smart glass.”